Congratulations on a successful surgery and an opportunity to keep on living. I commend you for wanting to ride. Whatever bike you get, also purchase a heart monitor and talk to your cardiologist about sustained (ideal) and maximum heart rates. He/she will give you the numbers. Also, start out easy. Don't know what you did in rehabilitation, so also ask about the frequency and length of each exercise on your bike. That, coupled with Dean Ornish's books, and a modified lifestyle of cooking/eating, should put you on a long road to living well.
Well done